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Need Help with ... P0171 and P0174 and Pinging ... Finally FIXED !!

4816 Views 15 Replies 7 Participants Last post by  paul r
Hi all,

Working on 2006 Sienna with 170 K miles.

Trying to solve codes P0171 + P0174 + Pinging under light to moderate acceleration issue.

Apparently, these codes indicate "Lean" conditions being reported to the ECU?
Also, the Lean conditions can contribute to Pinging.

Switching to Mid grade fuel helped a bit with the pinging.

From my research, I should be looking for:
1) Vacuum hose leaks
2) Air Leaks after the MAF
3) Test for Low Fuel Pressure.

I have cleaned the MAF sensor and made no change.
No obvious Air Leaks or Vacuum Leaks have been found.

Code reader shows both P0171 and P0174 in both Active and Pending lists.
After reset the same codes return within 10 miles of driving.

Ideas please??

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Most likely cause is air getting in through a vacuum leak. The problem with vacuum leaks is that 75% of the leaks can't be seen through a visual inspection. There are some DIY options (i.e. starter fluid spray on the likely leak locations hoping to hear RPM increases, but they can be hit or miss. A smoke test is really the best option, if the visual inspection reveals nothing. When doing the visual, make sure you also check the various hose clamps, including on the intake boot and verify there are no missing/severed hoses. Unless you're a "real" mechanic, you likely don't have access to a smoke tester, so I'd probably call around. If you're a DIYer and recently did something like change the plugs without replacing the intake and throttle body gasket, they would be prime suspects. Other than that, a failure in the fuel system could create low pressure to make the system lean, but I feel that is a far less common failure point in our Siennas. If they both showed up at the same time, it's likely not the A/F sensors, but if you had an intermittent code that you kept clearing hoping it would go away on it's own and now you have two codes, then those would also be potential considerations for failure.
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BillG .... TY for your help with this problem. No work has been done on the engine etc. so its not an unconnected hose or something left off and visually everything looks ok and with spraying starter fluid etc produces no changes either?

A clue perhaps? About a year ago I was getting P0420 and P0430 codes that now no longer show up just the P0171 and P0174 together with Light to moderate Pinging on acceleration. Is it possible that my AF sensors have gone bad? Should I "Clean" them if that is possible?

One other thing .... the "Flexible Joint" in the exhaust has deteriorated and the last half inch or so is missing and broken away from the attachment to the downstream exhaust pipe. Could that be the cause of the codes?

Is it time to "Pay" to have it Read by a Professional Diagnostic System to accurately determine the source of the codes?

Anybody have ideas ... Thanks!
Correct me if I am wrong but the flex joint in that year is basically as it comes under the engine just about directly above the cabin? So basically your exhaust system is wide open at that point?
Yes Sir, .... the "Mesh Flex Joint" is about 6 inches long and the last half inch of the Mesh is missing.

1) Could this be the "Cause" of all the "Codes"? 🙄 Front Bank is to the left in the pic.

2) Is there any way for me to "Repair" this without welding?

Automotive tire Wood Artifact Sculpture Gas
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I had this exact joint fail on our 06 years ago. I would never go to the dealer since I know they probably would have replaced everything and charged thousands. I went to a local exhaust shop, they said easy fix and it was. I swear they didnt take any more than 10 minutes to cut out the old one and weld in a new one. Couldnt really tell it wasnt OEM they did such a good job, and it was only $170. Keep in mind that was quite a few years ago but I doubt it would be a whole lot more.
Would that cause your codes and pinging? I have no idea because mine was just a small leak when I had it fixed, but what I can say is this is a dangerous spot for a leak. The last thing you want is exhaust gases coming up into the passenger compartment and that is exactly where this joint sits. So if you ask me its something you want to get done and asap, and since you need to get it fixed anyways, it seems like a first step I would take in a heart beat. Get this fix first, clear the codes and see if they come back. Worst case scenario it doesnt fix your codes but at least you know you wont be breathing in carbon monoxide anymore.
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I would double check the hose from the air cleaner to the throttle body.

Look for rodent damage to vacuum lines. For both banks to be 'lean' hopefully a easy fix.

When in doubt look again. I don't think the exhaust leak is the problem.
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3Wheelerguy .... I will Double and Triple check the hoses .... TY
Yes Sir, .... the "Mesh Flex Joint" is about 6 inches long and the last half inch of the Mesh is missing.

1) Could this be the "Cause" of all the "Codes"? 🙄 Front Bank is to the left in the pic.

2) Is there any way for me to "Repair" this without welding?
1) Possibly. Everything wants to perform with everything else intact. When you open something up that's supposed to be closed, it can cause cascade failures. However, in this particular case, probably not the cause of both banks reporting lean AND pinging. You're running lean and it's probably because of air getting in somewhere.

2) Not really. The curved pipe makes it hard to get a good clamp repair on there. You could potentially replace the whole section, but it's usually about $150-250 for this repair from any muffler shop or indie mechanic to cut and weld it for you. The part looks to be about 1/2 that price for aftermarket or double that price for factory original part and then you'd have to do the work yourself and deal with rusty exhaust bolts.
3Wheelerguy .... I will Double and Triple check the hoses .... TY
I have a 2016 Gen 3. This was the reason for my three trouble codes. There was a very slight separation, that was not very discernible at first, between the hose from the throttle body to the air cleaner box where the MAF sensor is located. It seems the hose clamp on the air cleaner side was not clamped securely.

Good luck!

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Very "Exciting Progress" on this issue!!

I want to go through a few more "Drive Cycles" before declaring it fixed.

I'll post details mid week ....
It's Fixed!!

Thanks for all of the input and help brainstorming this issue.


  • The fix required only one new part. I did also replace the "Air Filter" since, in an extreme case, it can contribute to a "Lean" mixture if very very clogged but it made no real difference in my case.
  • It's been a long time since my Code Reader reports "NO CODES". Still getting use to that!

  • NO MORE PINGING on light to moderate acceleration!!
  • Drivability is much much improved. No more hesitation or missed or delayed shifts
  • I'll be going back to "Regular" grade gas instead of "Plus" since it is not required to combat the pinging. That alone will, over time, pay for the part!!

THE BAD ....

- Now that the "Lean" condition is gone, my gas mileage has gone down maybe by 5% or so. Kind of sucks but makes sense because the "Lean" condition I was fighting meant "Stingy on fuel".


  • 170,000 miles on still original plugs, no engine management sensors have ever been replaced.
  • I had been having consistent codes P0420 and P0430 codes starting about a year ago (did not affect drivability) but then they disappeared several months ago and the P0171 and P0174 codes became persistent.
  • After checking and rechecking the hoses and lines for air leaks I simply could not find any.
  • Scanned and checked codes and cleared codes dozens of times over many months and once, very recently and only once, I got two unusual codes ... P0102 and P0113 ... which gave me the courage to start replacing parts!
  • P0102 and P0113 can be related to the MAF sensor, so I took a chance and ordered a DENSO MAF for about $65 off Amazon.

THE FIX ....
- Apparently, the entire problem and all of the codes over the past year or more was the MAF Sensor!!

  • My gut feeling is that the MAF Sensor has been slowly failing for 10's of thousands of miles now, resulting in the variety of codes. Cleaning the MAF made no difference. NOTE: The fine sensor wires inside the old MAF looked noticeably crusty and tired compared to the pristinely clean wires in the new MAF.
  • I got lucky ... One "Plug and Play" part and a Fix for less than $100!!

Thanks for the help
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Thanks for reporting back ! This really makes the forum work!!
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OMG Mr. 2006-SIENNA you have helped me solve my problem. I have been chasing p0171 and p0174 and loss of power at high RPM for months now. My LT fuel trims increased with increased engine load which would normally be a failing fuel pump and not a vacuum leak (long FT would get better with higher load), and I was about 2 inches from going the fuel pump route until I read your post. I had cleaned my MAF before, but I took it out and looked at it and noticed the crusty pickup wires just as you described. Looks like it was starting to fail but not enough to throw any MAF codes. I dropped a new DENSO one in there, and BOOM! instant fix. Fuel trims are perfect again. It even fixed my pinging problem just as you had! You sir are a lifesaver.
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You sir are a lifesaver.
Well now, this is how a forum should work. Glad you fixed your issue and please drop in again.
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